Date of Award
Doctor of Philosophy (PhD)
The ε4 allele of the apolipoprotein E (APOE) gene is the strongest genetic risk factor for late-onset Alzheimer disease (AD), a neurodegenerative disorder that leads to cognitive dysfunction and dementia. One copy of APOE ε4 increases AD risk by 3.7-fold and two copies by 12-fold, whereas APOE ε2 is protective relative to the more prevalent ε3 allele. APOE4 accelerates the progression of AD by markedly impairing amyloid-β (Aβ) clearance and promoting excess Aβ aggregation, ultimately resulting in downstream neuroinflammation, tau pathology, and neurodegeneration. Aggregated Aβ can co-deposit with the APOE protein in the brain parenchyma as neuritic plaques and in cerebral blood vessels as cerebral amyloid angiopathy (CAA). The presence of pathogenic APOE may act as a “seed” to promote Aβ fibrillization, and our lab has previously shown that reducing APOE protein levels with several genetic approaches in mice results in drastically less Aβ pathology. These findings imply that modulating APOE levels, particularly APOE4, directly influences Aβ deposition and may consequently alter downstream AD neuropathology.
Passive immunotherapy is a relatively feasible strategy that has been tested preclinically in mice and can be clinically translated in humans. Currently, AD clinical trials using immunotherapy center on targeting different species of Aβ and tau for degradation. Given that reducing APOE levels can directly modify Aβ plaque deposition, our lab produced antibodies targeting murine APOE, which demonstrated efficacy in both prevention and treatment studies. These promising findings prompted us to ask whether antibodies that bind human APOE, which have translatable implications, could similarly improve Aβ pathology. We generated a series of anti-human monoclonal APOE antibodies (HAEs) with varying properties, including binding affinity for different APOE isoforms, lipidation status, and Fc receptors. One antibody of particular interest was human anti-apoE antibody (HAE)-4. HAE-4 preferentially recognizes poorly-lipidated, aggregated APOE3 and APOE4 found in the core of amyloid but not circulating in plasma. Treatment with HAE-4 in APPPS1-21 mice that express human APOE4 at the onset of parenchymal plaque pathology resulted in Aβ reductions. The efficacy was dependent on antibody effector function for the recruitment of activated microglia, as ablating Fc binding negated any positive effects. Overall, these exciting results suggest that HAE-4 has potential as a therapeutic candidate for the treatment of amyloidosis in AD. However, one of several pressing questions that resulted from this study remains: how do APOE antibodies compare to Aβ antibodies that are currently in clinical trials?
To address this question, it is important to understand that certain Aβ antibodies that are now being tested are highly effective in reducing amyloid in clinical trials and are now beginning to show potential effects on slowing cognitive decline, though the effects appear modest presumably due to late intervention at a timepoint when events downstream of Aβ have been initiated. However, these amyloid-removing anti-Aβ antibodies also result in cerebrovascular adverse effects that are stimulated at high drug doses, namely amyloid-related imaging abnormalities (ARIA). ARIA is a common neuroimaging finding that is associated with antibodies targeting aggregated Aβ in individuals with CAA. ARIA manifests as edema (ARIA-E) or hemorrhages (ARIA-H) and is thought to result either from accelerated antibody-mediated clearance of Aβ exiting the perivascular space or from a strong, focal immunological response to binding CAA. ARIA is sometimes asymptomatic, but a subset of individuals develops headache, confusion, and focal neurological signs that are usually reversible. While optimism remains high for Aβ immunotherapy, there is some caution about its generalizability if nearly all AD patients also have CAA, particularly if the presence of CAA in combination with certain Aβ antibodies is responsible for ARIA.
Owing to the affinity of HAE-4 to engage only a small but critical core component of APOE within Aβ plaques, whereas many anti-Aβ antibodies bind to the majority of the protein in plaques, we hypothesized that APOE antibodies have a somewhat different mechanism of action compared to Aβ antibodies. We directly compared the effects of HAE-4 to aducanumab, an Aβ antibody that reduces plaque load in the brain but also induces ARIA in human clinical trials. Overall, our aims were to investigate the underlying mechanisms of a novel monoclonal antibody targeting APOE, its ability to ameliorate plaque pathology in the vasculature, and to what extent CAA-compromised blood vessel function could be restored. To address these questions, we used 5XFAD mice expressing human APOE4+/+ (5XE4) that have prominent CAA and parenchymal plaque pathology. In chronically treated 5XE4 mice, HAE-4 reduced Aβ deposition including CAA compared to a control antibody, whereas aducanumab had no effect on CAA. Furthermore, aducanumab exacerbated microhemorrhage severity, which strongly correlated with reactive astrocytes surrounding CAA. In contrast, HAE-4 did not stimulate microhemorrhages and instead rescued CAA-induced cerebrovascular dysfunction in leptomeningeal arteries in vivo. HAE-4 not only reduced amyloid but also dampened reactive microglial, astrocytic, and proinflammatory-associated genes in the cortex. These results suggest that targeting APOE in the core of both CAA and plaques could ameliorate amyloid pathology while protecting cerebrovascular integrity and function. Whether HAE-4 exerts its effects through indirect clearance of Aβ plaques or removal of poorly-lipidated APOE, which may have a gain of toxic function to promote further seeding of fibrillar plaques, remains a question to be addressed in future studies. Taken together, our findings argue a paradigm shift away from an Aβ-centric treatment strategy and towards targeting APOE to treat AD or CAA.
Chair and Committee
David M. Holtzman
John R. Cirrito, Celeste M. Karch, Jin-Moo Lee, Andrew S. Yoo,
Xiong, Monica, "Apolipoprotein E Immunotherapy for the Preclinical Treatment of Alzheimer Disease and Cerebral Amyloid Angiopathy" (2021). Arts & Sciences Electronic Theses and Dissertations. 2474.